Boost ASIO synchronous UDP client/server

If you know how to write an app that uses an ASIO TCP connection, you are close to know also how to do it on UDP.

Large part of the differences are taken care for us in ASIO, and we just have to use the socket as defined in boost::asio::ip::udp instead of its tcp counterpart.

Server

First thing, we create a udp socket, that requires the ASIO I/O context and a udp endpoint, that needs as parameters the IP protocol to be used - version 4 or 6 - and the port - here I picked up 50013.

namespace ba = boost::asio;
namespace bs = boost::system;
using ba::ip::udp;
// ...

const unsigned short HELLO_PORT = 50'013;
// ...

void server(ba::io_context& io)
{
    udp::socket socket{ io, udp::endpoint{udp::v6(), HELLO_PORT} };
 // ...

Then we repeat how many times we like this block - in my tester I did it just once:

std::array<char, 0> recvData;  // 1
udp::endpoint endpoint;  // 2
bs::error_code error;  // 3
socket.receive_from(ba::buffer(recvData), endpoint, 0, error);  // 4
if (error)
 throw bs::system_error(error);  // 5

std::string message{ "UDP hello from ASIO" };

bs::error_code ec;
socket.send_to(boost::asio::buffer(message), endpoint, 0, ec);  // 6

1. In this buffer we store the message sent from the client. It has no use here, so it could be it even zero sized.
2. The endpoint, that will be used to sent the message to the client, is set by the socket receive_from() method, two lines below.
3. This error code is set by receive_from(), in case of problems.
4. The server wait synchronously here for the client. The three parameters are output ones. When the connection starts, the data coming from the client is put in the first parameter (here, an empty message is expected), the second parameter is filled with the client endpoint, the last one stores the possible error in the operation.
5. If receive_from() fails, throw the boost system error code relative exception, that is a standard runtime_error subclass.
6. Send the message to the client, using the endpoint as set by receive_from() and not specifying any flag. Any possible error code returned is disregarded.

Client

The client function tries this code:

udp::resolver resolver{ io };
udp::endpoint destination = *resolver.resolve(udp::v6(), host, HELLO_PORT_STR).begin();  // 1

udp::socket socket{ io };
socket.open(udp::v6());  // 2

std::array<char, 0> sendData;
socket.send_to(ba::buffer(sendData), destination);  // 3

std::array<char, 128> recvData;  // 4
udp::endpoint sender;
size_t len = socket.receive_from(ba::buffer(recvData), sender);

std::cout.write(recvData.data(), len);  // 5

1. Having instantiated a udp resolver on the previous line, we resolve() on it for the same IP protocol of the server - here I used version six - specifying its host and port. Since resolve() returns at least one endpoint or fails, we could safely access the first one dereferencing its begin() iterator.
2. We need an ASIO upd socket. Having created it on the previous line, passing the current ASIO I/O control, we open it for the required UDP version.
3. We start the communication with the server, sending an empty message - as nothing more is expected from it.
4. We'd better have enough room for the message coming from the server, the actual size of it is returned by the call to receive_from().
5. Let's see what we get, outputting it to the console.

Client and server are two free functions in the same C++ file that I pushed to GitHub. Passing no parameter to its main you run it as server, otherwise is a client.

This example is based on Daytime.4 and Daytime.5 from the official Boost ASIO tutorial.